We intend to carry out the proposed research along these different paths: 1) The development of the sinoatrial node for voltage clamping in our lab makes it possible to examine the ionic mechanisms responsible for generation of diastolic depolarization. We shall examine the effects of Cl, pH, and Cl blockers on this tissue. We will also examine the K ion fluxes using K-selective electrodes under these ionic manipulations and conditions. 2) The kinetics and magnitude of slow inward current will be studied in frog ventricular muscle. The effect of new Ca 2 ion-antagonists developed recently on the size and kinetics of this current will be examined. Experiments are planned to examine whether the channel which carries the slow inward current behaves as a conventional H- and H-type of channel, or whether the slow channel may counter transport cations. 3) Experiments are planned to examine in great detail the inotropic kinetic effects of K ion on development of tension. The object of this section of the study will be to determine whether the inotropic effects of K ion on heart muscle are mediated thoough the alterations of Na ion-pump activity or whether the effect is mediated directly through an effect on Ca 2 ion transport system which may be mediated to K ion efflux.